1. Field of the Invention
The present invention relates to optical fiber gratings, and in particular to an improved temperature-compensating structure for optical fiber gratings.
2. Technical Background
Optical fiber gratings are important elements for selectively controlling specific wavelengths of light within an optical fiber. Such gratings include fiber Bragg gratings and long-period gratings.
Fiber Bragg gratings are optical waveguide fiber devices that selectively reflect specific wavelengths of light propagating in an optical waveguide fiber. Fiber Bragg gratings consist of a plurality of perturbations in the index of refraction spaced along the fiber length. These perturbations selectively reflect light of wavelength xcex equal to twice the spacing xcex9 between successive perturbations times the effective refractive index, i.e., xcex=2neff xcex9, where xcex is the vacuum wavelength and neff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for fiber dispersion.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance xcex9xe2x80x2 which is large compared to the wavelength xcex of the transmitted light. In contrast, with conventional Bragg gratings, long-period gratings use a periodic spacing xcex9xe2x80x2 which is typically at least 10 times larger than the transmitted wavelength, i.e., xcex9xe2x80x2 greater than 10 xcex. Typically, xcex9xe2x80x2 is in the range of 15-1500 micrometers, and the width of a perturbation is in the range of ⅕ xcex9xe2x80x2 to ⅘ xcex9xe2x80x2. In some applications, such as chirped gratings, the spacing xcex9xe2x80x2 can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast, with conventional Bragg gratings in which light is reflected and stays in the fiber core, long-period gratings remove light without reflection by converting it from a guided mode to a non-guided mode. A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. The spacing xcex9xe2x80x2 of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength xcexp from a guided mode into a non-guided mode, thereby reducing in intensity a band of light centered about the peak wavelength xcexp. Alternatively, the periodicity xcex9xe2x80x2 of the long-period fiber grating may be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode) which is substantially stripped off the fiber to provide a wavelength dependent loss. Such devices are particularly useful for equalizing amplifier gain at different wavelengths of an optical communication system.
In Bragg gratings, both neff and xcex9 are temperature dependent, with a net temperature dependence for a grating in a silica-based fiber typically having an average value of about 0.0115 nanometers per degree C at a wavelength of 1550 nanometers over the temperature range of interest in optical communication systems. It should be noted that temperature dependence is nonlinear. The temperature-induced shift in the reflection wavelength typically is primarily due to the change in neff with temperature. The thermal expansion-induced change in xcex9 is responsible for only a small fraction of the net temperature dependence of a grating in a conventional silica-based fiber.
Similarly, long-period gratings also exhibit high temperature sensitivity. The peak wavelength xcexp shifts by 5-15 nanometers per 100xc2x0 C. change in temperature. This sensitivity is about 5 times higher than for fiber Bragg gratings. Over the ambient temperature range experienced by optical communication systems such variation is not acceptable.
Accordingly, various methods have been devised for compensating for the undesirable performance effects caused by ambient temperature fluctuations. Thermoelectric heaters/coolers may be used for maintaining the optical grating at a desired temperature to prevent temperature-induced shifts in operating wavelength. However, active temperature control using heaters/coolers is expensive, and often impractical. Accordingly, passive temperature compensation techniques are highly desirable.
Various passive temperature-compensating structures have been devised that regulate the amount of tension on the portion of an optical fiber containing an optical grating to compensate for changes in the temperature of the optical grating.
One method of achieving passive temperature compensation employs tension adjustment of the portion of the optical fiber containing the optical grating. This method involves fixing the fiber to a substrate material having a negative coefficient of thermal expansion, such as xcex2-eucryptite. As the temperature increases, the substrate contracts thereby maintaining the reflective wavelength of the grating. A disadvantage with this technique is that xcex2-eucryptite requires hermetic packaging in order to function reliably over the range of environmental conditions specified for optical communication systems. Another disadvantage is that the hermetically sealed negative expansion substrate package is undesirably large.
Another method of passive temperature compensation utilizes materials of dissimilar thermal expansion characteristics to form a composite substrate to which the fiber grating is attached. Typically, the fiber is attached to the substrate at two points, with the grating located between the two attachment points. Because of the differences in thermal expansion of the two dissimilar materials, the layered composite substrate bends and the distance between the two attachment points decreases as temperature increases, thereby reducing the strain of the grating and thus compensating for the temperature-induced changes in the optical characteristics of the grating.
Another method that utilizes adjustment of the tension on an optical fiber containing an optical grating uses a mounting device comprising an arrangement of two materials of greatly differing coefficients of thermal expansion. The fiber is mounted to the device so that the amount of tension on the fiber decreases as temperature increases. Thus, the thermal expansion and thermally induced refractive index change of the grating are compensated for by the release in fiber strain.
A problem with the known temperature-compensating devices employing members made of materials having different coefficients of thermal expansion to provide a mounting having an effective coefficient of thermal expansion that is negative, whereby tension on a fiber mounted to the device is relieved with increasing temperature, is that the temperature-compensating effect does not provide a truly athermalized grating (i.e., the optical characteristics of the grating are temperature independent). Instead, the temperature- compensated gratings exhibit a center wavelength shift at most temperatures, that can range up to about 30 to 50 picometers. It is anticipated that future performance requirements of optical communication systems will require substantially improved athermalization for passively temperature-compensated optical fiber gratings.
It has been discovered that improved temperature compensation for an optical grating can be achieved using a temperature-compensating structure comprising a plurality of members that are selected and arranged to provide an effective coefficient of thermal expansion that is increasingly negative with increasing temperature with respect to two mounting points for an optical fiber grating. The improved performance is achieved by using a temperature-compensating structure wherein at least one of the members has a coefficient of thermal expansion that increases with increasing temperature and is higher than the coefficient of thermal expansion of at least one other member which has a coefficient of thermal expansion that decreases with increasing temperature.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.